The present invention relates to igniters such as are used in rocket engines, jet engines and combustors in general; and to igniters utilizing a hot surface, heated by acoustically heated gas, as an ignition source in particular.
Generally the safest, most reliable and most widely used method of igniting a combustor which does not employ a pilot light is an electrical spark. This approach is generally reliable and safe, e.g., such as used in an internal combustion engine spark plug. Historically, however, igniters for rocket engines have often used a pyrotechnic igniter or hypergolic ignition to assure reliable engine ignition. Ignition is particularly a concern in liquid rocket engines where both the fuel and oxidizer are supplied as liquids to the chamber, because any momentary delay in ignition can result in the accumulation of an explosive mixture of fuel and oxidizer, resulting in a hard start which may damage or destroy the engine. Restartable rocket engines are often necessary where the engine is used to perform orbit circularization, orbital maneuvers, or orbital transfer. Multiple pyrotechnic igniters, one for each use of the engine, have been used. Reusable engines also require multiple starts, and, while replaceable pyrotechnic igniters are possible, they may leave residues which may add to the cost of reconditioning the engine for re-flight. Another approach to reliable ignition is to use propellants which are hypergolic (ignite on contact with each other) so that multiple restarts of the engine are not generally a problem. Hypergolic fuel combinations are widely used in rocket engines employed in missiles, rocket boosters, and/or maneuvering engines, in large part because they provide a simple and reliable ignition process. Non-hypergolic propellant combinations in rocket booster stages often use a limited quantity or slug of hypergolic propellant in one or both of the propellant lines or are separately injected into the combustion chamber to initiate combustion. In such a case multiple starts become complicated. Although engines utilizing hypergolic propellants readily perform multiple restarts and are widely used, using hypergolic propellant combinations limits propellant choice and can limit performance. Moreover, generally hypergolic propellants are themselves expensive and toxic, such that the cost of procurement and handling may be significantly increased as compared to non-hypergolic propellants.
Electric spark ignition has been used to overcome these problems particularly with the hydrogen and oxygen propellant combination such as on the Pratt & Whitney RL 10 engine. Hydrogen and oxygen are clean burning, require low ignition energy, and have wide flammability limits. However, electrical ignition sources add complexity, require electrical power and a high-voltage electrical source, and are susceptible to electromagnetic damage such as caused by lightning strikes, and generally provide low ignition energy.
One possible ignition source which has been considered particularly for hydrogen and oxygen propellants is an acoustic igniter. An acoustic igniter employs a nozzle which directs an under-expanded sonic or supersonic gas jet into an essentially blind hole which forms an acoustic resonance tube. This arrangement, originally used as a high frequency noise source, was subsequently investigated as a simple way of obtaining a small quantity of very hot gas, or a hot surface which can be used as a source of ignition.
What is needed is a practical acoustic resonance igniter for H2 and O2, particularly with relatively low pressure gasses.